1. Field of the Invention
This invention relates to a method for minimizing bulk vortex pinning in granular Type II superconductors, and to the resulting superconductors. The method has been applied to a preferred superconducting granular aluminum system to determine the optimum fabrication conditions for minimizing vortex-pinning in this material. This invention further relates to the application of these low pinning materials to superconducting electronic devices which depend on vortex motion for their operation, permitting in particular the construction of a unity-coupling direct current transformer which is free of heating instabilities and capable of distortion-free operation over a wide dynamic range of operating conditions.
2. Description of the Prior Art
It is known in the art that a number of Type II superconducting devices depend on vortex motion for their operation, including for example thin-film microbridge applications such as quantum interference magnetometers and frequency mixers, as well as magnetic flux coupled devices such as the direct current transformer originally disclosed and claimed in U.S. Pat. No. 3,394,317 issued to I. Giaever. A primary problem, however, with such devices is that the Type II superconducting materials from which they are usually fabricated have vortex pinning forces that are typically very large. These large vortex pinning forces in turn necessitate operation of such devices at very high current densities. In the case of thin film microbridge applications this can lead to heating instabilities which drive the superconducting film into the normal state. In the case of the direct current transformer, as will be explained further, this restricts operation of the device to a very limited current range and necessitates operation of the device under conditions wherein the transformer's secondary voltage is a non linear function of the primary voltage.